Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The slider is coupled with a suspension that supports both the body of the slider and a head assembly that has a magnetic read/write transducer or head or heads for reading/writing information to or from a location on the disk. The complete head assembly, e.g., the suspension, slider, and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. There are tracks at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable hard drives such as those used in personal digital assistants (PDAs), Moving Picture Experts Group audio layer 3 (MP3) players, and the like. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters of less than 2.5 inches. Advances in magnetic recording are also primary reasons for the reduction in size.
However, the small drives have small components with very narrow tolerances. Disk drive sliders are designed to fly in very close proximity to the disk surface. For instance, in some systems the slider may be designed to fly only three to five nanometers above the disk surface. Moreover, in many hard disk drives, there is a disk stack and usually a read/write head associated with each side of a disk in the stack.
In many cases, to reduce cost, a single suspension will be used between the top of a first disk and the bottom of a second disk, the suspension will usually have a top slider with a read/write head and a bottom slider with a read/write head. Thus, a single suspension will be able to be utilized between the two disk surfaces at a great savings in cost and complexity.
However, one problem with utilizing a single suspension is the offset between the read/write heads and the two disk surfaces. For example, if the top read/write head is a bit closer to the disk, then the bottom read/write head may be a bit too far away, or vice-versa. These variations in the magnetic spacing between the head element and media are likely to cause data errors—both hard errors during writing and soft errors during reading.
This shortcoming becomes even more deleterious as the actual offset distances and overall components are reduced in size.